1. Field of the Invention
This invention relates to automated ultrasonic inspection systems, and is particularly directed to apparatus for pulsing a piezoelectric transducer from a remote station while performing in-service inspection of nuclear reactor coolant systems and the like.
2. Description of the Prior Art
Automated ultrasonic in-service inspection systems have been developed in the recent past for fast breeder reactors and both pressurized and boiling water reactors. The technical approach which has been followed features remotely controlled traveling instrument carriers, and computerized collection and storage of inspection data in a manner providing real time comparison against predetermined standards. The radiation levels experienced in components other than the reactor during in-service examination require that automated inspection systems be operated from a main control console which operates an instrument carrier from a remote position as far as 300 feet.
The philosophy adopted for the computer controlled instrument carrier has been to maintain as much as possible of the computer control electronics at the remote console. In practice, the instrument carrier (commonly referred to as a "skate") is provided with only enough structure to permit positioning the skate along a track adapted to fit the specific contour of the steam generator, reactor vessel, and the like, to be inspected. The details of the skate and track, and a method of mounting the track are disclosed in a copending U.S. application Ser. No. 248,466, filed Apr. 28, 1972, by Laurence S. Beller for an INSPECTION ANALYSIS SYSTEM, now U.S. Pat. No. 3,857,052 assigned to the assignee of this application.
In order to minimize the size and weight of the skate, only the piezoelectric transducer of the ultrasonic inspection system is mounted on the skate. The piezoelectric transducer is connected to the main control console through a flexible transmission line. The transducer is then periodically pulsed under control of the computer through the transmission line, and echoes sensed by the transducer following each pulse are carried back to the main computer console through the same transmission line.
In the past the automated ultrasonic inspection systems have used two separate, but similar techniques to pulse the ultrasonic transducer through the transmission line. In one technique, the piezoelectric transducer capacitance is placed in parallel with an input coil to form an LC tank circuit tuned to the resonant frequency of the transducer. This tuned circuit is triggered into resonance through the transmission line with a high-voltage pulse generator. For example, in one transducer pulsing arrangement, a capacitor of up to 2,400 picofarads of capacitance is charged to 1,000 volts at the remote console and discharged through a thyratron into the transmission line. In another arrangement three 330 picofarad capacitors are charged to 400 volts in parallel and discharged into the transmission line in series.
The problem with these prior art techniques is that a tuned circuit is relied upon to apply a voltage pulse to the transducer of consistently high amplitude for a consistent period each time the transducer is to transmit an ultrasonic wave. If the amplitude and duration of the applied pulse is not consistent, the transmitted ultrasonic wave will not be consistent, the consequence of which is that echoes received by the transducer will also not be consistent for a homogeneous body being inspected. It is, of course, evident that any inconsistencies in the received echoes will be interpreted as a variation in the body (vessel wall or other structural member) being inspected. Such variation would render the inspection data unreliable.
Serious problems are present in these techniques for pulsing a tuned circuit through a transmission line because precise tuning of the resonance must take into consideration the capacitance of the transmission line, and therefore its length. When a very long coaxial cable is used, a very large capacitance in the cable is connected to the tuned tank circuit. For example, 300 feet of RG59/U add 6900 picofarads of capacitance to the tuned circuit. The coil of the tuned circuit must be slug-tuned to the resonance frequency of the transducer, typically 2.25 MHz. This type of tuning for the tank circuit can only adjust for approximately 1,500 picofarads of capacitance. The tank circuit may also be adjusted by a variable capacitor within the adjustment range of 7.5 - 100 picofarads. In either case, the input cable is treated solely as a capacitance, and adding or subtracting short lengths of cable is taken care of by the tuning adjustment. However, when the length of the input cable is increased beyond a certain point (about 100 feet), it is no longer to be considered as a pure capacitance, but as a 75 ohm transmission line with a transit time of 1.5 nanoseconds per foot. Consequently, for a 300 foot cable, the transit time for a pulse is 450 nanoseconds, and any mismatches at either end of the line can cause reflections. Any reflection from the drive end back to the transducer would interfere with echo signals being received through the transmitter.
In these prior art techniques, the pulse generator is transferring the energy stored in a capacitor at the remote station of the main control console to the cable and the tuned circuit at the skate carrying the piezoelectric transducer. The efficiency of this energy transfer depends on the impedance transform of the circuit. Changing circuit values to increase the excitation voltage applied to the piezoelectric transducer can adversely affect other properties of the tuned circuit, such as repetition rate and pulse shape.
To avoid these problems of the prior art, it would appear to be necessary to place the pulse generator on the skate carrying the transducer for cable lengths in excess of about 100 feet. However, this solution is not feasible since the added pulse generator would increase the size and weight of the load on the skate so that it would then be necessary to provide a larger skate and heavier drive system for the skate. Another disadvantage is that an operator would not be able to make adjustments on the pulse generator to accommodate a longer output cable for the transducer, and an impedance match would still have to be made.